“Aerosols” is the name used below to mean, in general, mixtures of solid and/or liquid suspended particles (also referred to in general as “particles” below) and gaseous media, for example, in particular air. Aerosols are meant to be, in particular, aerosols with particles in the micrometer range, that is to say in the range <1000 and/or, even preferably, in the nanometer range, that is to say in the range <1000 nm.
Examination and characterization of aerosols plays an important role in various areas of natural sciences, technology, medicine and daily life. By way of example, the surface characterization of aerosols and aerosol particles plays a critical role in the fields of environmental analysis and medicine, since the surface distribution and the surface morphology of aerosols can have a decisive influence on, for example, the toxicity of nanoparticles and, for example, the assessment of workplace pollution caused by aerosols and nanoparticles.
Knowledge of the structures of the particles, in particular of agglomerate structures, is indispensable for assessing workplace pollution of nanoparticles, parameterizing the inhalation-toxicological potential and process control in the synthesis of gaseous nanoscale particles. The on-line observation of particle formation is likewise of great interest in many other fields, for example, in meteorology and climate research or aerosol physics.
In particular, gas-borne nanoscale particles, i.e. particles having a size of, for example, <1000 nm, or else micrometer particles, i.e. particles having a size of, for example, <1000 μm, are often in the form of agglomerates or aggregates, i.e. sintered agglomerates, of so-called primary particles. The structures of the agglomerates are here, for example, loosely linked in the manner of a chain and/or branched, or may even be spherically sintered.
To characterize the particles or aerosols, a large number of different devices and methods have been developed which enable, on-line or off-line, important statements about characteristics of the particles to be made. As described herein, “off-line” measurements are measurements in which the measurement is effected independently of the flow, for example, with time displacement and/or in a separate apparatus. In contrast, “on-line” measurements are those which are carried out directly and without any major time displacement, for example, real-time measurements or measurements which are carried out at least nearly in real time.
The detection and counting of such particles already play an important role in characterization processes, in particular in the field of nanoparticles. A large number of different types of particle counters are known and available commercially and are based on different measurement principles. For example, one measurement principle is based on detection by way of light, for example, laser light. An example of such a laser particle counter is disclosed in WO 91/08459. Other particle counters or particle detectors for ultra small particles are based on charge effects, for example, a particle counter disclosed in WO 2007/000710 A2. Other on-line measurement techniques, such as those based on scattered light methods (for example, scattered laser light), are also known. Other counters and detectors are based on electrostatic principles, such as the particle sensor disclosed in WO 2007/000710 A2. It is also possible to use so-called condensation nucleus counters or condensation particle counters (CPC) in particular to be able to detect even very small particles, for example, particles in the lower nanometer range, which is comparatively difficult using conventional light techniques. In these counters or detectors, the size of the particles is artificially increased, for example, by way of depositing a film of condensate comprising, for example, butanol, such as by providing a condensate sleeve around these particles. The particles whose sizes are increased in this manner can then be detected comparatively easily. U.S. Pat. No. 4,790,650 discloses an example of a condensation particle counter.
Besides the pure detection and the counting of particles, classification, in conjunction with a corresponding detection of the particles, also plays a role. Conventionally, the particles are classified in an electrodynamic manner by categorizing the particles in accordance with their mobility, that is to say the ratio of the velocity of the particles to the force acting on the particles, into classes or fractions. In the case of electrically charged particles, in particular the so-called electrical mobility (often also referred to as Z) is used, i.e., the ratio of the velocity of the particles to the electric field acting on the particles.
The mobility of a body moving in a liquid or a fluid (gas or liquid) is usually expressed by the so-called mobility diameter dm, which is frequently also referred to as mobility-equivalent diameter. This is the diameter of a fictitious sphere which has said mobility in the fluid (for example, the carrier gas used).
A large number of devices and methods have been developed for the classification, or, in other words, for separating the particles, for example, in accordance with their mobility. One example of such a device is the so-called differential mobility analyzer (DMA). These analyzers are generally variable electric filters which, for example, as a function of variable or fixedly pre-specified geometrical dimensions and/or of variable or fixedly pre-specified electric voltages, only allow particles of a specific electrical mobility from a particle flow to pass. Examples of such differential mobility analyzers are disclosed in WO 2007/1016711 A1. Classifiers of this type are frequently connected to corresponding counters which directly allow the number or concentration of particles in the specific, filtered-out class to be counted. It is possible in this manner, for example, to determine concentrations and particle size distributions of the totality of the particles by changing the class. Such instruments are referred to, for example, with minor structural differences, as “DMPS” instruments (differential mobility particle sizers), SMPS (scanning mobility particle sizers) or FMPS (fast mobility particle sizers). Examples of such classifier systems, which are connected directly to measuring instruments or counters, are disclosed, for example, in U.S. Patent Publication No. 2006/0284077 A1, in U.S. Patent Publication No. 2004/0080321 A1, in GB 2378510 A, in GB 2374671 A, in GB 2346700 A, or in WO 99/41585.
Since charging particles or particle flows plays an important role in many methods or devices known in the art, a large number of devices have been developed which can produce defined charges on the particles. These devices, also referred to below as “charge state generators” or “chargers,” can produce, for example, specific charge distributions (for example, probabilities that a particle accepts one, two or more positive and/or negative elemental charges) or a fixedly pre-specified number of such charges on the particles. An example of such devices is disclosed in EP 1 678 802 A2, in WO 00/78447 A1 (there in connection with a DMA and a CPC), or in DE 198 46 656 A1. If the same number of positive and negative charges are produced, such charge state generators are frequently also referred to as neutralizers, such as is disclosed, for example, in U.S. Pat. No. 6,145,391.
As described above, in the on-line characterization of particles, in particular aerosols, spherical equivalent particle sizes are generally assumed. This is, for example, a foundation of the above-mentioned DMPS, SMPS and FMPS methods, since the mobility diameter dm is always used. However, this could potentially result in significant errors if the ascertained characteristic values are further used. By way of example, different types of agglomerates cannot be differentiated. In addition, the error in the diameter also comes into play in the volume calculation of the particles to the power of three and thus also, for example, the mass calculation of the particles (if the density is known). The resulting errors in the determination of the mass concentration are significant. The inaccuracies of the known methods and devices also become very noticeable in the calculation of the particle surface areas, in which the errors in the diameter come into play to the power of two. This is a significant disadvantage of the known methods and devices, in particular in the field of toxicology, where the surface areas and surface distributions of the particles play a significant role. In addition, shape factors, in which, for example, the differences between rod shape, spherical shape, plate shape or similar shape differences come into play, can hardly be detected using the known methods.
Therefore, the on-line determination of the primary particle diameter, of the number of primary particles per agglomerate particle and of the shape factors of the agglomerates and other structure-specific parameters can overall hardly be carried out using the commercially available measurement methods. In order to determine those parameters, off-line measuring methods are conventionally used, in which some of the particles are taken from the totality, for example, by way of samplers, in order to introduce them into other characterization methods. By way of example, these other characterization methods can be imaging characterization methods, for example, scanning electron microscopy (SEM), transmission electron microscopy (TEM), or atomic force microscopy (AFM). Examples of samplers of this type, with which samples can be taken from the totality, are disclosed, for example, in WO 2004/009243 A1 or in JP 2007127427 A. The off-line methods described are, however, expensive and time consuming, and, in particular, do not permit on-line characterization and/or control, based on the evaluation of the characterizations, for example, of process parameters, manufacturing parameters, or safety measures in the field of protection at workplaces.
Further approaches for solving the above-mentioned problems of the particle diameters are based on the fact that the particle diameters are determined, rather than a metrology method, on the basis of charging theories and theories relating to drag forces acting on agglomerates. An example of such a theoretical or semi-empirical method can be found in “On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis: I. Theoretical analysis,” Aerosol Science 37 (2006) 260-271 by A. A. Lall et al. and in “On-line measurement of ultrafine aggregate surface area and volume distributions by electrical mobility analysis: II. Comparison of measurements and theory,” Aerosol Science 37 (2006) 272-282 by A. A. Lall et al. The model described therein combines a mobility analysis, carried out by means of a DMA or an SMPS, with calculations relating to the drag force acting on agglomerates and the charging efficiency of agglomerates. A theoretical approach is used which is based on a large number of assumptions which are restrictions at the same time. For example, it is assumed that the agglomerates comprise primary particles. The latter must be spherical and have a primary particle size which is already known in advance. Furthermore, the surface of the agglomerates must be accessible. This means that primary particles do not cover each other which, for example, rules out aggregates having primary particles which are clearly fused together. Such a method can therefore not be applied to partially sintered agglomerates (aggregates). Overall, the described model therefore comprises a large number of model-based restrictions and assumptions which must be met so that the model provides realistic results.